The UNSC is attempting to drive the Storm Covenant off of Requiem. Crimson team was sent down to push buttons and save eggheads and eventually they found the Didact's Gift, which teleported Glassman to the Covenant place. Jul 'Mdama was there, and he commands Glassman to help them revive the Librarian AI, or Gek will blow the scientist up.
Unsure on what to do, Lasky brings Catherine Halsey aboard, who discovers that the Didact's Gift is scanning them, and begins receiving text messages from an unknown entity, who is revealed to be Jul 'Mdama. Thorne offers to babysit Halsey and learn about the Spartan-IIs. Meanwhile, Castle Team is killed by 'Mdama's forces. Crimson rescue more eggheads and at Galileo Base they find a Promethean Soul. Majestic Team is sent to defend it, but when the Covenant attack they are forced to leave.
Back at the Infinity, the Promethean Soul is revealed to have the memories of a human who was Composed at New Phoenix. Halsey receives a message asking her about the Librarian, which surprises her, prompting Lasky and Palmer to lock her away again. 'Mdama is angered by this, but turns to find Glassman finally activating the device. All the Elites bow down, and 'Mdama is happy.
On the Infinity, Thorne touches the Gift, and is teleported to the Cauldron, where he is surrounded by Elites.
I may have missed stuff out, but that's what I remember.